Plasmas, which are more or less ionized gases, are electrically conductive fluids which may have a wide range of energy densities and electron temperatures. Plasmas have thus found extensive use in industrial processes involving cracking, dissociation and deposition as well as gas polishing. Examples of industrial processes employing plasmas include i.e. thin film deposition, plasma chemistry, plasma spray and bulk materials work, materials synthesis, welding, fusion etc.
Plasma properties are usually measured in terms of density (electrons per cubic meter) and electron temperature (which may be measured in K or electron volts). The latter being a direct measure of the degree of ionization, i.e. that proportion of the atoms that have lost an electron. Plasma density and temperature can vary considerably; density from 10−3 to 10+30 particles per cubic meter and temperature from 0 K to 10+8 K. Plasma lifetimes are also an important measure and may also have very wide range, typically from 10−12 to 10+17 seconds. It can therefore be seen that the term “plasma” can represent an extremely wide range of conditions, and for any particular application, it is important to specify the type of plasma being used.
Various forms of plasma are known to exist, generally categorised by their energy characteristics: principally thermal plasmas and non-thermal plasmas.
Thermal Plasmas (TP)
Thermal plasmas have electrons and heavy particles (ions and neutrals) at the same temperature, i.e. they are in thermal equilibrium. Thermal plasmas are readily produced, for example by electric arc, and so are easily scalable from a few tens of watts to several megawatts. Typically, they require high energy input for given reaction result as the entire mass of plasma is heated with associated thermal losses and problems of handling and containment.
The effect of thermal plasmas is to simply increase the total energy content and weight average temperature of the process products. As a result, a new equilibrium composition of the components is established according to the plasma energy contribution, whose effect in this case is quantitatively identical to the effect of a thermal energy contribution of the same value.
Non-Thermal Plasmas (NTP)
Non-thermal plasmas are not in thermodynamic equilibrium, thus the effect of the plasma results in a thermodynamically non-equilibrium composition of the process products. Usually the ion temperature in NTP's is different from the electron temperature, the electrons being ‘hotter’ than the heavy particles. For this reason, NTP's are also referred to as “cold plasma” or “non-equilibrium plasma” in the literature.
NTP's may be produced using a number of techniques, including electrical discharge in a vacuum (barrier discharge) capacitive and inductive coupled plasmas, as well as radio-frequency (RF) and microwave electromagnetic methods.
While thermal plasmas can operate at any pressure, NTP's prefer to operate at low or near vacuum conditions, some forms only operate at low pressure, while others, such as microwave produced plasmas can operate at high pressure (atmospheric).
NTP's at low pressure are relatively easy to create in larger volumes and to initiate as the damping effect of surrounding heavy particles is minimized, however, the plasma density is also limited, thus limiting their commercial value (residency times need to be greater). High pressure NTP's operating at near or above atmospheric pressure are continually damped by the proximity of the surrounding heavy particles (atoms) and so require greater formation energy. However, a high intensity plasma results in a more versatile and commercially viable plasma reactor as residency time is short and continuous operation is possible. Thus, strong non-equilibrium, high energy high pressure plasmas which may be obtained by microwave non-thermal plasmas are desirable.
Microwave Non-Thermal Plasmas
Microwave NTP's are particularly effective for plasma chemistry because they require relative low energy input to form highly reactive plasmas due to low thermal losses and strong catalytic effects of the high electron temperatures. Also, NTP's give no contamination from the electrode when they are made by electrode-less nozzle designs.
Microwave NTP's are notably difficult to produce in homogenous volumes, unlike barrier discharge systems that can be designed over larger areas, however, microwave NTP's are more efficient in terms of energy coupling but are constrained by the size of the microwave source (magnetron). Magnetrons operating with power exceeding 1 kW in the GHz frequency range are able to maintain the steady state microwave discharges at atmospheric pressure. At low and intermediate pressures the plasma is strongly non-equilibrium—the temperature of the neutral component (Tg=300 K) is less than the electron temperature (Te˜1-2 eV).
Most of the large-scale plasma chemical applications require high power and high pressure for high reactor productivity. It is also important to have a high degree of non-equilibrium with high electron temperature and density to support selective chemical processes. Therefore, it is necessary to have a powerful discharge that generates non-equilibrium plasma for chemical applications with both high efficiency and selectivity.